Recently, active research is being carried out into wireless power transfer using magnetic induction in a low frequency band. However, the method using magnetic induction is disadvantageous in that power can be transmitted only within a short range of a few centimeters. Furthermore, there are many difficulties in applying this method using magnetic induction to wireless power transfer systems because it has very low efficiency when the arrangements of Tx and Rx coils are not identical with each other.
Korean Patent No. 10-0809461 discloses a configuration which is capable of increasing the power receiving distance by using an electromagnetic amplification relay employing LC resonance. In this patent, a method is used of performing LC resonance using a variable capacitor in a solenoid-type coil in which an induction coil is wound on a magnetic body to increase magnetic flux. This method uses separate LC resonant coils in Tx and Rx units, unlike the existing configuration used for magnetic induction. In this patent, the distance and efficiency of power transfer can be increased, as described above. In the invention disclosed in this patent, the resonant frequencies of the Tx and Rx power resonant coils are tuned using the variable capacitor. However, this invention has a disadvantage in that it is difficult to precisely adjust the value of the variable capacitor to a value which matches the resonant frequency. Furthermore, this preceding patent discloses wireless power transfer using only a parallel arrangement between the resonant coils, and therefore it is difficult to put this prior patent to practical use in a variety of ways.
Furthermore, U.S. Patent Application Publication No. US 2009/0224856 A1 discloses a wireless power transfer method using magnetically coupled resonance. This U.S. Patent discloses the general details of a magnetically coupled resonance method, and discloses elements related to the Q factor and the resonant frequency. This patent presents a scheme for improving power transmission efficiency and transmission distance using a magnetic resonant structure having the same resonant frequency and very strong magnetic coupling.
In U.S. Patent Application Publication No. US 2009/0072629 A1, resonant coils are constructed using a variable capacitor by means of a method similar to that of a Korean patent (Korean Patent No. 10-080941).
U.S. Patent Application Publication No. US 2009/0153273 A1 proposes a method of improving transmission distance and efficiency by adding additional resonant coils between Tx and Rx resonant coils that are arranged coaxially with the Tx and Rx power resonant coils as well. However, this method relates to a serial arrangement made by taking into consideration higher coupling constant between the Tx and Rx power resonant coils, and all the resonant coils are on the same plane. If all the resonant coils are on the same plane, the coupling constant between the resonant coils is reduced, and transmission efficiency is deteriorated.
The four conventional patents do not describe the arrangement of the coils, and attempt a method of improving power transfer efficiency and transmission distance on the assumption that all the coils are in a parallel arrangement (that is, axis of each coil is identical with one another). If only the parallel arrangement is used, however, there is the need for a solution capable of solving difficulties that occur when the parallel arrangement is applied to real life because of spatial limitations. Furthermore, in order to freely situate a power receiving unit, there is the need for a method that enables power transmission even in a vertical arrangement or various arrangements between the transmitting and receiving coils, but there has been no solution for such a method.